The Dialiinae clade represents one of the early-diverging lineages of the legume subfamily Caesalpinioideae; it consists of 17 genera (circa 90 species), and is pantropically distributed. Morphologically, the group comprises a diverse assemblage of taxa that may represent a so-called “experimental phase” in legume floral evolution. Different members of the clade exhibit poly-, mono-, and asymmetry, as well as having undergone a high degree of organ loss, producing, in some cases, extremely reduced flowers which are barely recognisable as belonging to the legume family.
In order to obtain a clearer picture of floral evolution in the Dialiinae, a well resolved and well supported phylogeny is needed onto which morphological characters may be optimised. With the goal of creating such a phylogeny, a total of 37 Dialiinae DNA samples were sequenced for two plastid genes, rpS16 and trnL. Additionally, a comprehensive morphological study was carried out. A total of 135 vegetative and reproductive characters were scored for 79 ingroup and four outgroup taxa. Phylogenetic analyses were carried out first on a restricted group of taxa for which all three data sets were available. The highly supported nodes of this phylogeny were then used as a constraint for a second parsimony analysis of morphological data from a much larger taxon set. Morphological characters were then mapped onto one of 20,000 most parsimonious trees from this second analysis. A number of novel species-level relationships were resolved, creating a clearer picture of changes in floral form over time, particularly in the genera Labichea and Dialium.
In addition to their diverse mature floral morphology, the Dialiinae are also widely variable in their floral ontogeny, displaying both organ loss and suppression, and exhibiting a wide variety of organ initiation modes. In order to build a more complete picture of floral development and evolution in this clade, the floral ontogeny of several previously undocumented species was investigated. Complete developmental series were compiled for six species of the Dialiinae; four from Dialium, as well as Poeppigia procera and Mendoravia dumaziana. Mode and timing of organ initiation were mostly consistent across the Dialium species studied. In the instances of both gains and losses of floral organs in Dialium, one trend is apparent — an absence of abaxial organs. Gains in both sepals and stamens occur in the adaxial median position, while stamens and petals that are lost are always the ventral-most organs. The taxa examined here exemplify the apparent lack of developmental canalisation seen in caesalpinioid legumes. This ontogenetic evolvability is reflective of the morphological diversity shown by flowers across the subfamily.
One of the species of the Dialiinae, Apuleia leiocarpa, produces an andromonoecious inflorescence, a feature that is unique in its clade and rare in the Leguminosae as a whole. Light and electron microscopy were used to undertake a detailed study of the floral morphology of this taxon. It was found that while hermaphrodite flowers produced a single carpel and two stamens, staminate flowers developed three stamens but showed no sign of carpel development. Inflorescences also appear to produce approximately four times as many staminate as hermaphrodite flowers, with hermaphroditic flowers consistently occupying the central position in cymose inflorescences. Both this high male-to-bisexual ratio and the early determination of gender seen in Apuleia are rare in the Caesalpinioideae and suggest that andromonoecy developed in this genus as a means to increase pollen dispersal rather than in response to resource limitations.